This project is focussed on the development of strategies for the treatment of solid tumors. The antineoplastic investigations revolve around the development of a complete understanding of the mechanisms of action and resistance to the antimetabolite class of agents. These studies have identified that the treatment of patients and malignant cells in in vitro model systems with fluoropyrimidines and antifolate agents results in an acute induction of thymidylate synthase and dihydrofolate reductase respectively. Since the level of intracellular thymidylate synthase is an important determinant of sensitivity to fluoropyrimidine agents, the acute induction of this enzyme following fluoropyrimidine exposure is a central cause of resistance to these agents both in vivo and in vitro. Interferon gamma has been shown to repress the acute thymidylate synthase induction resulting from fluoropyrimidines. This repression results in enhanced sensitivity of malignant cells. While the precise mechanism of interaction between interferon represents an ongoing investigative effort, we have identified an unprecedented mechanism of autoregulatory translational control as a means by which cells regulate the intracellular levels of thymidylate synthase. In addition to its catalytic function it has become apparent that thymidylate synthase also plays an important regulatory role in cellular biology. Recent investigations have identified an interaction between thymidylate synthase protein and the messenger RNA's of the myc family of oncogenes. Given the importance of thymidylate synthase as a chemotherapeutic target, we have developed sensitive assays for the quantitation of this enzyme in cells and human tissues using a monoclonal antibody directed against thymidylate synthase. The use of monoclonal antibodies has resulted in ultrasensitive detection of thymidylate synthase, quantitation of enzyme free and bound by fluoropyrimidines and quantitation on a per cell basis in human tissues and cells. The availability of these sensitive assays will help delineate the role of thymidylate synthase as a prognosticator of survival and response. Recent investigations have demonstrated a close association between the level of thymidylate synthase expression and responsiveness of gastric carcinoma to fluoropyrimidine containing regimens. Furthermore, investigations have defined the level of thymidylate synthase expression in primary tumors of patients with rectal carcinoma to be a significant and independent prognosticator of disease free survival and survival in this patient population.